Conventionally, a variety of clamping devices for locking a workpiece to be machined on a workpiece fixing table such as a workpiece pallet, have been proposed or are in practical use.
Among these, a clamping device for drawing and locking a workpiece to the table is described, for example, in Laid Open German Patent Publication DE-4020981-A1. As shown in FIG. 5, the clamping device 100 described in this publication has a clamping body 101 in which a pull rod 102 is provided to be vertically movable and protrudes upwardly. The pull rod 102 is provided at the top end with a tapered rod portion 102a having a tapered shape to increase the horizontal cross-sectional area toward the top. A collet member 103 is fitted on the upper part of the pull rod 102. The collet member 103 has a collet 103a fitted onto the tapered rod portion 102a, which is elastically deformable in a diametrically expanding manner.
The bottom end of the pull rod 102 is coupled to a piston 105 of hydraulic cylinder 104, which is able to drive the pull rod 102 downwardly. In addition, the pull rod 102 is biased upward by a spring 106.
On the other hand, the collet member 103 is supported by a collet supporting member 108, that is resiliently biased upward by coned disc springs 107.
For locking a workpiece W, hydraulic pressure is applied to the hydraulic cylinder 104, with the tapered rod portion 102a and collet 103a being inserted into a vertical bore Wa formed in the workpiece W. As a result, the pull rod 102 is driven downwardly to lower the tapered rod portion 102a. Since the collet member 103 is biased upward by the coned disc springs 107 via the collet supporting member 108, the collet 103a is slightly lowered and elastically deformed in a diametrically expanding manner along with the downward movement of the tapered part portion 102a, so that the collet 103a engages in the vertical bore Wa, and the workpiece W is drawn and locked to the workpiece receiving surface 101a of the clamp body 101.
In the clamping device described in Laid Open Japanese Patent Publication H11-188551, similar to above described clamping device, a collet member is supported by a collet supporting member from below, with the supporting member being biased upward by a coil spring. The pull rod is vertically driven by a double action hydraulic cylinder.
Problems to be solved by the present invention will be described hereafter.
In the clamping device of the former patent publication, the collet member is resiliently biased upward by the coned disc springs, which generally yields easily. Once coned disc springs yield, it is not assured that the collet is resiliently deformed in a diametrically expanding manner to lock the workpiece. Inevitably, the clamping device has reduced durability.
On the other hand, in the clamping device of the latter patent publication, the collet member is resiliently biased upward by a coil spring. Coil springs may also yield, but they are less likely to do so than the coned disc springs. However, it would be difficult to say that high durability is obtained thereby. Furthermore, a double acting type hydraulic cylinder is used to move the pull rod vertically to lock and unlock a workpiece. Hence, the hydraulic system must be switched each time a workpiece is locked and unlocked, complicating the switching operation.
The purpose of the present invention is to improve the durability of a clamping device and to facilitate the locking operation by using a hydraulic cylinder to bias a collet member upwardly.